In circuit breaker drawout installations where the circuit breaker is equipped to afford overcurrent and ground fault circuit protection, separate phase current transformers, utilized to individually sense the currents flowing in the respective phase buses of the circuit to be protected, are typically mounted integrally with the breaker and their secondary windings permanently wired to a static trip unit, also integrally mounted with the breaker. For circuits utilizing neutral buses, a separate current transformer must be utilized to sense current flowing in the neutral bus if the existence of a ground fault is to be properly recognized. This neutral current transformer, located externally of the breaker in embracing relation with the neutral bus mounted within the switchboard, must also have its secondary winding wired into the breaker static trip unit. Since, in drawout installations, the breaker is mounted for movement relative to the switchboard between engaged and disengaged positions, the circuit connection between the trip unit and the neutral current transformer must be made via trailing cable of sufficient slack to accommodate this breaker movement. Within the congested confines of a switchboard, the accommodation of a trailing cable in a manner to prevent fouling can be difficult. Moreover, repeated flexure of the trailing cable can produce wire breakage.
An alternative approach to completing a circuit between the neutral current transformer and the drawout breaker trip unit is to wire the transformer secondary to a stab connector mounted by the switchboard. A complementing stab connector, mounted by the breaker, is wired to the trip unit. When the breaker is racked into its engaged position, these two stab connectors mate or plug together, completing the circuit between the neutral transformer and the breaker trip unit. To insure proper mating, these stab connectors must be relatively mounted in rather precise alignment, requiring close manufacturing tolerances. Also of concern is the fact that while the stab connectors are unmated, the neutral transformer secondary winding is open-circuited. Current then flowing through the switchboard neutral bus can induce extremely high voltages in the secondary winding capable of breaking down winding insulation. Moreover, this high secondary voltage condition poses an extreme, hidden hazard to operating and maintenance personnel.
It is accordingly an object of the present invention to provide an improved disconnect for making and breaking a circuit between the secondary winding of a switchboard mounted current transformer and a drawout circuit breaker mounted trip unit incident to movement of the breaker between its engaged and disengaged positions with respect to the switchboard.
An additional object is to provide a disconnect of the above character, wherein the disconnect parts respectively mounted by the switchboard and the breaker accommodate considerable degree of misalignment and breaker skew.
A further object is to provide a disconnect of the above character which includes switch means automatically operative, when the transformer is disconnected from the trip unit, to prevent the secondary winding from being open-circuited, and thus to preclude the development of a high voltage condition therein.
Another object of the present invention is to provide a disconnect of the above character which is inexpensive to manufacture, convenient to install, and reliable in operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.